During the process of fabricating integrated circuit devices, there are redundant circuits and other optional circuit arrangements which can be selected to assure desired operating functions. Such selections can be made by trimming certain fuses included in the device. Generally, a laser beam is used for trimming the fuses. An electrical test procedure is accomplished in conjunction with the laser trimming process.
As shown in the sequence of FIGS. 1(a) through 1(h), there is shown a somewhat uneconomical prior art method for making laser repairs. In FIG. 1(a) fuses 20 are fabricated in a polysilicon conductive layer that is surrounded by silicon dioxide 22. Bonding pads 24 are fabricated in a metal layer, which is covered by a layer of silicon dioxide 26 and a layer of silicon nitride 28, as in FIG. 1(b). A pix coating 30, e.g., a polymide resin, is laid over the silicon nitride patterned, and cured to provide access to the pads and fuses, as in FIGS. 1(c) and (d). The pix coating, the silicon nitride, and the silicon dioxide are etched away from the pads for direct access for electrical testing, as shown in FIG. 1(e) and l(f). The pix coating, the silicon nitride, and a part of the silicon dioxide are etched away from above the fuses for laser trimming, as shown in FIG. 1(g). An advantage of this process is that only a single photoresist mask step is used in the sequence of processing steps. Thereafter the electrical tests and the laser trimming are performed, but there are device moisture barriers 35 to prevent contamination caused by ambient atmospheric conditions. These moisture barriers 35 occupy a substantial area in addition to the circuit area and therefore raise the cost of making the device. The remaining partially exposed fuses may suffer corrosion or other deleterious effects from the exposure causing the device to subsequently malfunction. Molding compound 38 which is applied thereafter does not protect the device from harmful effects of the ambient atmosphere.
In another prior art method, test fuses 40 are fabricated in a polysilicon conductive layer surrounded by silicon dioxide 42 similar to the first method, as shown in FIG. 2(a). Thereafter a cap oxide 44, e.g., silicon dioxide, is deposited over the metallic layer 46 and silicon dioxide 42. The cap oxide is sintered and the surface of the device is patterned to nearly expose the fuses 40 of the device, as shown in FIG. 2(b). The cap oxide also is etched away from over the pads 46. The silicon dioxide is etched down nearly to the level of the fuses. In FIG. 2(c) electrical tests are performed through probes 48 contacting with the exposed pads, and a laser beam 50 is used for trimming fuses, as shown in FIG. 2(d). Once the testing and laser trimming are completed, a silicon nitride layer 52 is deposited over the top of the device and into the fuse cavity, as shown in FIG. 2(e). A pix coating 54 is laid over the silicon nitride 52 and patterned to provide access to the pads, as shown in FIG. 2(f). The pix material and the silicon nitride are etched to expose the pads for use in a system, as shown in FIGS. 2(g) and 2(h). Bonding wires 56 are connected to the bonding pads 46, and a molding compound 58 is added to encapsulate the device. A disadvantage to this method is that a second costly masking sequence is added into the sequence of processing steps. An advantage in this process is that the silicon nitride protects the non-trimmed fuses from deleterious effects which can occur if those fuses are left unprotected from the ambient atmosphere.